The present invention relates to a method and an arrangement for weighing batches of liquid and other pourable or flowable substances, particularly in industrial processes requiring high weighing accuracy.
In the weighing methods know from the prior art, the supply of pourable substance to be weighed into the container wherein it is to be weighed is effected by means of supply devices, such as vibratory conveyors, screw conveyors, valves or the like, which have the purpose of maintaining the flow of material to be weighed as uniform as possible. Upon receipt of a signal, the further supply of material to the container is stopped. Prior to reaching of the intended batch weight, the supply of material can already be throttled or a supply device can be used which has a lesser weight of feed, so that the amount of material which enter the receptacle from the supply device, i.e., such amount which issues from the supply device to enter into the receptacle, after the further supply has been shut down, remains small.
The reaction of the material flow (the impulse of the material flow) is factored into the weighing indication. If the material flow is constant, this reaction force can be compensated for only if it is less than the weight which enters into the weighing receptacle after the material shutdown as a result of residual outflow from the feed devices. In the event that the reaction forces exceed this weight value, actual weighing values are obtained upon completion of the inflow of material into the weighing receptacle and after the weighing system has reached equilibrium, which are smaller than the predetermined desired batch value. The exception to this is if the shutoff of the weighing system takes place only at a weight indication which is higher than the desired batch weight.
The aforedescribed influences upon the weighing system may in some cases be maintained within acceptable limits by limiting the maximum material flow and taking into consideration the existence of lead values.
However, very frequently the characteristics of the pourable material are such that the flow of material, that is the entry of material into the receptacle per unit time, is so strongly influenced by the characteristics of the pourable material, for example moisture, viscosity, temperature and the like, and by various other parameters, that it is de facto impossible to maintain the accuracy of the weighing systems within the required limits by setting fixed lead values.
In these cases, it is only possible to carry out a control weighing operation, interrupting the inflow of material before the desired batch weight has been reached, and thereupon to gradually work up to the desired batch weight by metered admission of small quantities of further material.
Such a weighing produces high weight accuracy, as will be readily understood, but it requires significantly more time and in many cases the time for the total weighing operation will be impermissibly long.